Structural, Magnetic and Dielectric Properties of Barium Cobalt ...

Journal of Solid State, Electronics and Devices Volume 1, Issue 2 www.stmjournals.com

Structural, Magnetic and Dielectric Properties of Barium Cobalt Hexaferrite-Powder and Barium Cobalt Hexaferrite-Epoxy Resin Composite Film C.C. Chauhan1, R.B. Jotania2* 1

Department of Electrical Engineering, Institute of Technology, Nirma University, Ahmedabad, Gujarat, India 2 Department of Physics, University School of Sciences, Gujarat University, Ahmedabad, Gujarat, India

Abstract The W-type barium cobalt hexaferrite powder with chemical composition BaCo2Fe16O27 was prepared using a simple heat treatment method. The precursor was calcinated at 650 °C for 3 h in a muffle furnace and then slowly cooled to room temperature in order to obtain barium cobalt hexaferrite powder. The flexible epoxy thin film of Ba-cobalt hexaferrite/epoxy composite was fabricated using the suitable amount of hardener and curing agent. Surface and morphological studies were carried out using X-ray diffractograms (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron micrography (SEM). The XRD pattern of hexaferrite reveals the formation of mixed phases– M and W. The FTIR spectra of hexaferrite powder samples were recorded at room temperature by using KBr pallet method. The field-dependent magnetic properties of the hexaferrite sample were investigated at room temperature using a vibrating sample magnetometer (VSM) with a maximum applied field of 15 kOe. The dielectric measurements were taken at room temperature within the frequency range to 100 MHz to 2 MHz.

Keywords: Ba-cobalt hexaferrite, heat treatment, curing agent, epoxy film, hexaferrite-epoxy resin composite film *Author for Correspondence E-mail: [email protected]

INTRODUCTION Hard Ferrites are technologically important materials that are used in the fabrication of magnetic recording, electronic and microwave devices due to their high electrical resistivity, high saturation magnetization and high magnetic permeability [1, 2]. Barium hexaferrites are magnetically hard magnetic materials that possess properties like high saturation magnetization, high coercivity, large uniaxial anisotropy and excellent chemical and magnetic stability. Due to these properties they are useful for numerous technical applications. They have been extensively used in permanent magnets, high density recording media and magneto-optical recording technologies, communication equipment and microwave devices [3–7]. The crystal structure of W-type hexagonal ferrite is very complex and can be considered as a

superposition of R and S blocks along the hexagonal c-axis with a structure of RSSR*S*S*, where R is a three-oxygen-layer block with composition BaFe6O11, S (spinel block) is a two-oxygen layer block with composition Fe6O8 and ‘*’ means that the respective block is turned 180° around the hexagonal axis. The conventional ceramic method for preparation of Ba hexaferrite is the solid-state reaction between BaCO3 and Fe2O3 at high calcination temperature (~1200 °C), which has inherent disadvantages such as chemical inhomogeneity, coarse grain size and then need to milling to get submicron sizes, which in turn leads to entrance of impurities and inducing lattice strains during the milling. To avoid this, various methods like the sol-gel [8], the chemical co-precipitation [9, 10], hydrothermal synthesis [11, 12], micro emulsion and reverse micelle [13, 14] and the

JoSED (2014) 1-7 © STM Journals 2014. All Rights Reserved

Page 1

Properties of Barium-Cobalt Epoxy Hexaferrite

Chauhan and Jotania

orgo-metallic precursor have been used. The authors have adopted simple heat treatment method to prepare W-type BaCo2Fe16O27 hexaferrite. Prepared barium cobalt hexaferrite powder was mixed with epoxy resin matrix with appropriate proportion of hardener and the curing agent to prepare the composite film. Experimental results of composite as well as hexaferrite powder samples are reported in present paper.

for 3 h for decomposition of the organic compounds and the crystallization of the particles. Prepared barium cobalt hexaferrite powder was mixed with epoxy resin (30% barium cobalt hexaferrite powder and 70% epoxy resin) to obtain barium cobalt hexaferrite-epoxy resin composite. Hexaferrite powder and ferrite-epoxy resin composite film samples were characterized using XRD, FTIR, VSM and dielectric measurements.

MATERIALS AND METHODS

RESULTS AND DISCUSSION

Highly pure salts of ferric nitrate [(Fe(NO3)2) 98% purity, Sigma Aldrihic], barium nitrate [Ba(No3)2 98% purity, Merck] and cobalt nitrate [(CoN2O6,6H2O)] were taken as starting materials. Barium cobalt hexaferrite powder was synthesized using metal nitrate reagent as precursors, polyvinyl pyrrolidone (PVP) as a capping agent and deionized water was used as the solvent. The aqueous solution of PVP was prepared by dissolving 3 g of PVP in 100 ml of deionized water at a temperature of 70 °C before mixing 1.6 mmol of ferric nitrate, 0.1 mmol of barium nitrate and 0.2 mmol of cobalt nitrate into the PVP solution and constantly stirring for 2 h on a magnetic stirrer until a homogeneous solution was obtained. The pH of solution was between 1 and 2. The mixed solution was heated at 80 °C for 2 h to evaporate the water. The resulting orange solid was crushed for 15 min and heated at 650 °C

X-ray Diffraction Analysis X-ray diffraction pattern of powder was recorded on a Bruker DZ Phaser X-ray diffractometer (PW 1830) using CuKα radiation (λ = 1.5405 Ǻ). Figure 1 shows XRD pattern of barium cobalt hexaferrite powder calcined at 650 °C. All XRD peaks were indexed using powder-X software. The XRD peaks position and intensity of diffraction lines were compared with standard JCPDS-file. XRD pattern of barium cobalt powder sample indicates both M and W phases. The unit cell of W-type phase is closely related to M-phase; the only difference is that the successive R blocks are interplaced by two S-blocks instead of one, as in the M-phase [15]. It was reported by Lotgering [16] that W-type hexagonal ferrite is chemically unstable; some of the Wphase gets decomposed to M-phase [16, 17]. The particle size obtained is 26 nm.

Fig. 1: XRD Patterns of BaCo2Fe16O27 Powder Calcinated at 650 °C Prepared Using Heat Treatment Method.


Page 2

Journal of Solid State, Electronics and Devices Volume 1, Issue 2

The lattice parameters calculated using the values of dhkl corresponding to the value of [206] peak were found to be a = 6.26 Å and c = 32.846 Å. The lattice volume of the sample V

3 2 a 2 c as

calculated using the formula 114.6 Å. The average crystalline size of the hexaferrite powder was calculated by the X-ray line broadening technique using Debye Scherrer formula using the profile of (206) peak. FTIR Analysis Figures 2(a), (b) show the FTIR spectra of powder and the composite film samples

respectively in a wave number ranging from 4000 to 400 cm−1. The absorption bands in barium cobalt hexaferrite powder sample appeared between 576 and 412 cm−1, which are attributed to stretching of Fe–O vibration. These absorption bands are suppressed by the addition of epoxy resin. The broad peak observed in ferrite-epoxy resin composite film between 3600 and 3200 cm−1, is due to stretching of O-H bond, while peaks between 2500 and 3000 cm−1 are due to carboxylic acids. The peak around 2100 cm−1 is due to C=C bond.

Fig. 2: FTIR Spectra of (a) BaCo2Fe16O27 Powder and (b) Ferrite-Epoxy Resin Composite. SEM Analysis

Fig. 3: SEM Images of (a) BaCo2Fe16O27 Powder and (b) Ferrite/Epoxy Resin Composite.


Page 3


Scanning Electron micrographs of prepared Ba-Co hexaferrite powder and composite film samples were obtained using a Make-Leo/Lica model Stereo scan 440 scanning electron microscope. The SEM image of BaCo2Fe16O27 powder (Figure 3a) shows the agglomerated particles of different shapes and sizes. The particles are found to be porous in nature, indicating that the hexaferrite particles of nano dimensions are formed. The SEM image of barium cobalt hexaferrite epoxy resin composite film shows the well dispersion of powder in polymer. Magnetic Measurements The magnetic property of barium cobalt hexaferrite powder and composite film have been measured at room temperature using a vibrating sample magnetometer (VSM) in a maximum applied field of 15 kOe. Figure 4 shows hysteresis loops of barium cobalt hexaferrite and barium cobalt hexaferrite/epoxy

Chauhan and Jotania

resin composite film. The saturation magnetization (Ms), remanent magnetization (Mr) and coercivity (Hc) were determined from the obtained hysteresis loops. The value of Mr (20.80 emu/g) is ~ 54% of the value of Ms (44.78 emu/g) for the normal sample. This result can be compared to the value found in literature and to that of the Stoner-Wohlfarth model [18]. The value of coercivity (Hc) is found to be 1.29 kOe. Hc can be correlated with the crystalline size and also with the method of preparation. It is very well possible that the change in the calcination temperature and time may produce samples with better Hc and Ms. The value of Mr/Ms, almost equal to 0.5, confirms the formation of single domain for both the samples. It is seen from Figure 4 that the addition of polymer composite has decreased saturation magnetization and the coercivity. Hysteresis loop is of S shape indicating the ferromagnetic behavior of ferrite.

Fig. 4: Hysteresis Loops of BaCo2Fe16O27 Powder and Ferrite/Epoxy Resin Composite. Specific Magnetization In order to decide Curie temperature of barium cobalt hexaferrite powder, specific magnetization measurement was carried out under max field of 10 Oe using low-field ac susceptibility instrument (Magneta). Figure 5

shows the variation of specific magnetization with temperature of barium cobalt hexaferrite powder. It shows standard behavior of hexaferrite. The Curie temperature (Tc) of prepared hexaferrite sample is found about 535 °C.


Page 4


Table 1: Room Temperature Magnetic Parameters of BaCo2Fe16O27 Hexaferrite Powder Prepared by a Simple Heat Treatment Method (Coercivity – Hc, Saturation Magnetization – Ms, Remanent Magnetization – Mr measured at 15 kOe). Sample

Ms (emu/g)

Mr (emu/g)

Hc (KOe)

Mr/Ms

BaCo2Fe16O27

44.78

20.8

1.29

0.46

BaCo2Fe16O27 + epoxy resin

18.52

9.07

0.96

0.49

Fig. 5: Variation of Specific Magnetization with Temperature for BaCo2Fe16O27 Ferrite Powder. Dielectric Measurements

Real Dielectric constant

80 without epoxy 14 with epoxy 12

60

10 8 6 4

40

2 0 1k

10k

100k

1M

20

0 1k

10k

100k

1M

Frequency (Hz) Fig. 6: Variation of Dielectric Constant with Frequency for BaCo2Fe16O27 Ferrite Powder. JoSED (2014) 1-7 © STM Journals 2014. All Rights Reserved

Page 5

Conductivity (ohm-cm)-1


6.0x10-7 5.0x10-7

Chauhan and Jotania

Without epoxy With epoxy

4.0x10-7 3.0x10-7 2.0x10-7 1.0x10-7 0.0 1k

10k

100k

1M

Frequency (Hz) Fig. 7: Variation of A.C. Conductivity with Frequency for BaCo2Fe16O27 Ferrite Powder. The dielectric measurements of the Ba-Co ferrite and the ferrite composite film were taken at room temperature in the frequency range 100 to 2 MHz (Figures 6 and 7). The dielectric constant is observed to be decreasing with increasing frequency for both powder and composite film. At high frequencies the dielectric constant is observed to be independent of frequency. This behavior of the samples can very well be explained in accordance with the Maxwell Wagner model [19, 20]. As per the model, the electron hopping between Fe2+ and Fe3+ does not follow the externally applied electric field, as a result dielectric constant decreases at a lower frequency and then it becomes constant at higher frequency. As per the Maxwell Wagner model, the dielectric structure of ferrite material is assumed to be made up of two layers – conducting layer which is made up of number of grains and the grain boundaries which acts as a poor conductor. Figure 7 shows the variation of ac conductivity with frequency.

The ac conductivity is observed to be increasing with increasing frequency. The conduction mechanism can be explained on the basis of hopping mechanism of Heikes and Johnson [21].

CONCLUSIONS The authors have successfully synthesized hexagonal BaCo2Fe16O27 ferrite powder and BaCo2Fe16O27/epoxy resin composite. BaCo2Fe16O27 ferrite powder exhibit the mixed phase (W and M-phase), but no traces of αFe2O3 was found. The curie temperature of the Ba-Co hexaferrite is observed about 535 °C. The behavior of dielectric constant follows Maxwell Wagner model. The electrical conductivity is explained by hopping mechanism between Fe+2 and Fe +3 ions.

ACKNOWLEDGMENTS This work was carried out under DRS-SAP program of UGC, New Delhi. The authors acknowledge Pondicherry University for providing them the magnetic measurements.


Page 6


REFERENCES 1. Upadhyay C, Mishra D, Verma HC, et al. Effect of preparation conditions on formation of nanophase Ni-Zn ferrite through hydrothermal technique. J. Magn. Magn. Mater. 2003; 260(1-2): 188–94p. 2. Cvejic Z, Rakic S, Kremenovic A, et al. Nanosize ferrites obtained by ball milling: Crystal Structure, cation distribution, sizestrain analysis and Raman investigations. Solid State Sciences. 2003; 8(8): 908–15p. 3. Koleva M, Atanasov P, Tomov R, et al. Pulsed laser deposition of barium hexaferrite (BaFe12O19) thin films. Appl. Sur. Sci. 2000; 154–155: 485–91p. 4. Sözeri H, Kućuk I, Ӧzkan H. Improvement in magnetic properties of La substituted BaFe12O19 particles prepared with an unusually low Fe/Ba molar ratio. J. Magn. Magn. Mater. 2011; 323(13): 1799–804p. 5. Carey R, Gago-Sandval PA, Newman DM, et al. The magnetic and magnetooptical properties of Co, Cr, Mn and Ni substituted barium ferrite films. J. Appl. Phys. 1994; 75(10): 6789–91p. 6. Yamauchi T, Tsukahara Y, Sakata T, et al. Barium ferrite powders prepared by microwave induced hydrothermal reaction and magnetic property. J. Magn. Magn. Mater. 2009; 321(1): 8–12p. 7. Pal M, Brahma P, Chakravorty D. Magnetic and Electrical properties of Nickel Zinc ferrites doped with bismuth oxides. J. Magn. Magn. Mater. 1996; 152(3): 370–4p. 8. Kabo O, Ido T, Yokoyama H. Properties of Ba ferrite particles for perpendicular magnetic recording media. IEEE Trans. Magn. 1982; 18(6): 1122–4p. 9. Zhong W, Ding WP, Zhang, et al. Key step in synthesis of ultrafine BaFe12O19 by sol gel technique. J. Magn. Magn. Mater. 1997; 168(1-2): 196–202p. 10. Jacobo SE, Domingo-Pascual C, Rodriguez-Clement R, et al. Synthesis of ultrafine particles of barium ferrite by chemical co precipitation. J. Mater. Sci. 1997; 32(4): 1025–8p.

11. Haneda K, Miyakawa C, Kojima H. Preparation of high coercivity BaFe12O19. J. Amer. Cera. Soc. 1974; 57(8): 354–7p. 12. Ataie A, Piramoon MR, Harris IR, et al. Effect of Hydrotheral synthesis Environment on the particle morphology, Chemistry and magnetic properties of barium hexaferrite. J. Mater. Sci. 1995; 30(22): 5600–6p. 13. Kumazawa H, Maedaand Y, Sada E. Further consideration of hydrothermal synthesis of barium ferrite fine particles. J. Mater. Sci. Letters. 1995; 14: 68–70p. 14. Pillai V, Kumar P, Shah DO. Magnetic properties of barium ferrite synthesized using a microemulsion mediated process. J. Magn. Magn. Mater. 1992; 116(3): L299–L304p. 15. Smith J, Wijn HPJ. Ferrites. Philips Technical Library, Eindhoven, 1959. 16. Lotgering FK, Vromans PHGM. Chemical instability of metal-deficient hexagonal ferrites with W structure. J. Amer. Cera. Soc. 1977; 60(9-10): 416–8p. 17. Jisheng K, Huaixian L, Youwei D. W type hexagonal ferrites RxBa1-xFe18O27. J. Magn. Magn. Mater. 1983; 31-34(2): 8012p. 18. Stoner EC, Wolhfarth EP. A mechanism of magnetic hysteresis in heterogeneous alloys. London: Philosophical Transactions of Royal Society of London. 1948. 19. Wagner KW. A Bibliography of Zimbabwean Archeology of 2005. J. Amer. Cera. Soc. 1913; 40: 817–23p. 20. Murthy VRK, Shobanadri J. Dielectric properties of some Nickel Zinc ferrites at radio frequency. Phys. Stat. Solid A. 1976; 36(2): 100–40p. 21. Heikes R, Johnson D. J. Chem. Phys. 1957; 26: 58p.


Page 7